As shown in FIG. 4, a conventional electromagnetic valve assembly 20 has a bobbin 23, a fixed core 21 including an input port 21aa valve seat 21ba magnetic movable valve member 27 including a cone part 27a, a bobbin 23, a non-magnetic sleeve 26, a cylindrical casing 22, a yoke 25, a plate 28 including an output port 28a and an electromagnetic coil 24. The non-magnetic sleeve 26 defines a fluid passage. The bobbin 23 is made of synthetic resin. The magnetic movable valve member 27 is movable into and out of engagement with the valve seat 21b disposed in the fluid passage. The fixed core 21 establishes a magnetic circuit with the casing 22 and the yoke 25. An electromagnetic coil 24 is wound around the bobbin 23 and generates the electromagnetic force in the magnetic circuit.
Accordingly, the movable valve member 27 is attracted to the fixed core 21 and makes a contact with the valve seat 21b. Consequently, the flow of fluid is cut off between the input port 21a of the fixed core 21 and the output port 28a of the plate 28. When the electromagnetic coil is de-energized, the movable valve member 27 is separated from the valve seat 21b by the fluid pressure applied to the input port 21a. Consequently, fluid flows between the input port 21a of the fixed core 21 and the output port 28a of the plate 28. A groove 27b is formed on the outer surface of the movable valve member 27 in order to connect both ends of the movable valve member 27 with each other.
However, in the conventional valve assembly, the valve seat 21b is formed on the fixed core 21 and the magnetic movable valve member 27 makes contact with the valve seat 21b directly. Therefore, even when the electromagnetic coil is de-energized, the cone part 27a of the movable valve member 27 will still be attracted continuously to the valve seat 21b of the fixed core 21 by the residual magnetism. The effect related to residual magnetism is further analyzed hereinafter.
The valve seat 21b and the cone part 27a are disposed coaxially with each other. However, it is very hard to arrange the cone part 27a coaxially to the valve seat 21b perfectly because of manufacturing errors in the dimensions. Accordingly, the cone part 27a tightly contacts the valve seat 21b at some part thereof, but is separated from the valve seat 21b at some other part thereof. Thus, the movable valve member 27 is attracted to the fixed core 21 by the residual magnetism at the contacted part between the movable valve member 27 and the fixed core 21, but is not attracted as strong at the separated part. Furthermore, the fluid supplied to the input port 21a does not flow through the contacted part but flows through the separated part.
As a result the fluid flows between the separated parts of the movable valve member 27 and the fixed core 21 where the attracting force due to residual magnetism is weakened. For example, the fluid flows along the path designated by arrow F in FIG. 3. Therefore, the cone part 27a is affected by the flowing fluid, and one part of the cone part 27a is forced against the valve seat by the attracting force of the residual magnetism as shown by arrow P. Further, the other part of the cone part 27a is forced away from the seat by the separating force shown by arrow Q. Thus the valve member 27 is subjected to a rotating force T. For example, as shown in FIG. 3, the valve member 27 is subjected to a clockwise rotating force T. Accordingly, the axis of the valve member 27 tends to be inclined relative to the axis of the sleeve 26 and become stuck or wedged in the sleeve. As a result the stuck valve member 27 cannot be moved by low fluid pressure and the valve member cannot be opened.